Research On Ppb-Level NO₂ Sensors Operating At Room Temperature Based On Zinc Oxide Composites

NO₂ is one of the common toxic and harmful gases in the environment,and the national Environmental Air Quality Standard GB3095-2012 stipulates that 100 ppb is the safety threshold for human exposure to NO₂.The rapid sensors to detect ppb-level NO₂ are crucial for environmental monitoring and human health.Resistance-type gas sensors based on semiconductor metal oxide（SMO）have attracted widespread attention due to their advantages of simple fabrication,all-solid state,low cost,and wide detection range.Among them,ZnO is one of the first materials discovered to have gas-sensing properties,and the detection of NO₂ at room temperature（RT）can be achieved through photoactivation.However,ZnO-based NO₂ sensors working at RT still have some shortcomings,such as slow response/recovery speed,insufficient detection limit and poor humidity resistance.The aim of this thesis is to develop room-temperature planar NO₂ sensors with fast response/recovery speed,low limit of detection（LOD）,and excellent humidity resistance,which is achieved through the design of ZnO-based composites and the implementation of sensitization strategies such as surface-defect regulation and morphology regulation.The main researches are as follows.（1）In order to improve the utilization of visible light energy,CsPbBr₃ quantum dots（QDs）with excellent photoelectric properties are modified on the surface of ZnO microspheres（MSs）,and the fabricated NO₂ sensor can work at RT.The gas-sensing materials are prepared by a three-step method.CsPbBr₃ QDs and ZnO MSs are synthesized by hot injection method and hydrothermal method,respectively,and then the composites are obtained by mixing them in different proportions.When the amount of CsPbBr₃ is 2.2 at%,the sensor has the best gas-sensing characteristics at RT under520 nm light（LED,the light energy is about 1.2 W/cm²）,with the response of 2.6 to 1ppm NO₂,the response and recovery time of 100 s and 107 s respectively,and the good selectivity and repeatability.By introducing humidity compensation,the change rate of the responses under different humidity is reduced from 98%to 16.1%.Finally,the adsorption/desorption experiments of NO₂ are carried out under different background gases,and it is found that NO₂ is mainly adsorbed by directly capturing electrons from SMO,proving that the oxygen vacancies（O_V）play an important role in the NO₂ sensing process.The modification of CsPbBr₃ QDs on ZnO improves the visible light absorption capacity of the composites so that the sensor can work under the activation of visible light,which enhances the surface activity of the composites and speeds up the response/recovery rate.（2）In order to solve the problem of insufficient LOD of the sensor in the previous work,two-dimensional（2D）BiOI with excellent photocatalytic performance is introduced into the gas sensing materials.The composites are designed by modifying BiOI on the surface of one-dimensional（1D）ZnO nanorods（NRs）,achieving ppb-level NO₂ detection at RT.BiOI-ZnO composites are prepared by a two-step method,and BiOI is grown directly on the surface of ZnO NRs by the hydrothermal method,which enhances the interfacial contact of the composites.After further sintering of the sensors in the Ar atmosphere,the contacts between the gas-sensing material and the electrode are much closer.When the amount of BiOI is 1.5 at%,the sensor has the response of14.5 to 1 ppm NO₂ under 520 nm light,and the response and recovery time of 66 s and47 s,respectively.The sensor can also realize the detection of 25 ppb NO₂,reach a theoretical detection limit of 0.34 ppb,and exhibit excellent selectivity and good stability.Besides,the ability of humidity resistance is also improved.This kind of sensor also exhibited the photovoltaic effect,which shows application potential in the field of photovoltaic self-power devices.After the modification of BiOI,the composites have improved light absorption capacity and the extended lifetime of the photogenerated carriers,which enhances the surface photocatalytic activity and thus realizes the high and fast responses of the sensor under visible light.Moreover,1D ZnO NRs have advantages such as good crystallinity and high aspect ratio,which further help improve the response/recovery speed.（3）2D ZnO nanosheets（NSs）modified by 2D SnS₂ with rich vacancy defects are created to realize the detection of ppb-level NO₂ at RT without the assistance of lights.First,the thicknesses of the SnS₂ layers and the content of sulfur vacancies（S_V）are adjusted by changing the sintering temperature,and it is found that SnS₂ sintered at 200℃showed good gas-sensing properties to NO₂.Based on the above results,SnS₂-ZnO composites are obtained by directly growing SnS₂ on the surface of ZnO NSs and sintering at 200℃.There are both S_V and O_V defects in the composites.When the amount of SnS₂ is 2.0 at%,the content of defects reaches the highest.At this time,the sensor shows the best gas sensing properties to NO₂ at RT,with the response of 10.5 to 1 ppm NO₂,and the response and recovery time of 85 s and 71 s,respectively,which can also achieve the detection of 50 ppb NO₂,and the theoretical LOD of 0.097 ppb.The sensor also has excellent selectivity,and improved humidity resistance.Besides,the recovery time will be further shortened to about 10 s by introducing the assistance of 520 nm light during the recovery stage.The sensor exhibits good gas-sensing properties to NO₂ at RT,related to the morphology of the gas sensing materials:（i）The N-N heterojunction formed at the interface between 2D-SnS₂ and 2D-ZnO will deepen the width of the depletion layer so that the depth of the depletion layer is very close to the thickness of NSs in the composite,and the resistance will change significantly,resulting in improving the responses of the sensor;（ii）Due to the synergistic effect of two different vacancy defects on the surface of the 2D-2D SnS₂-ZnO materials,there is competition for NO₂ adsorption,resulting in faster response and recovery speeds;（iii）Due to the weak interlayer binding energy of 2D-SnS₂,more NO₂ molecules may be adsorbed in the interlayer gap,and SnS₂ does not form hydrogen bonds with H₂O molecules,which hinders the adsorption of H₂O on the surface of the composites and improves the ability of sensor to resist humidity interference.